Ex-bourbon casks are American White Oak barrels that have previously been used to age – you guessed it – Bourbon.
When the barrel is first used, Bourbon absorbs a lot of the intense toasty, vanilla sweetness from the freshly charred wood, leaving less woody flavour compounds for the next spirit. When the Bourbon is emptied from the barrel, some of the bourbon spirit is left behind, absorbed into the wood, so some caramel honey sweetness from the corn sugars remains.
Meaning whisky aged in ex-bourbon barrels gains a subtle sweetness of vanilla, honey and caramel from the barrel.